Hairpin Circuit: Strange Spark Gap Behavior

[Kregus]

Hi,

After reading "The Inventions, Researches, and Writings of Nikola Tesla" and watching videos from Karl Palsness, I decided to build my own Tesla Hairpin/Lecher Line circuit.

>> Schematic & parts list <<
Schematics.com | Tesla Hairpin Circuit

The spark gap of this circuit works well in the following conditions:

1. When no capacitors are connected
2. When one capacitor is connected
3. When the copper short bar at the top is removed

However, in it's intended state, so with two capacitors and the copper short bar connected, the spark gap does not fire at all, no matter how large or how small I make the gap. See the video below for a demonstration.

[VIDEO]https://www.youtube.com/watch?v=_NBB2pVGxRQ[/VIDEO]

When I use two capacitors and connect the lamp to the copper bars, the lamp flickers once when I flip the ON switch, but then does nothing.

My question is: what could be the reason for my spark gap not firing when two capacitors and the short bar are connected?

Thanks in advance for the help!

[dR-Green]

My question is: what could be the reason for my spark gap not firing when two capacitors and the short bar are connected?

In the first test in the video, are the bars connected, or is it ONLY the spark gap?

The power supply frequency may be too high, and/or the capacitors may be too big.

If the power supply is AC, there's a short circuit across it through the capacitors, so if the capacitors are not small enough to filter the frequency then it won't work as you've got a short circuit = no spark. In that case you need capacitors that are small enough to filter/block the power supply frequency so it sees an open circuit, then it should produce a spark.

[edit] From the picture it looks like the power supply output is 14kHz, so you essentially need capacitors that are small enough to act as a high pass filter with a cutoff of at least 14kHz to allow the open circuit condition.

What happens if the capacitors are connected in series in test 2?

Also, you shouldn't be doing experiments on the floor when cats are walking around

[Kregus]

Hi dR-Green, thanks so much for your quick and detailed reply!

In the first test in the video, are the bars connected, or is it ONLY the spark gap?

Literally only the spark gap, I disconnected the leads to the caps in this test, and the caps lead to the bars, so they are automatically also disconnected.

The power supply frequency may be too high, and/or the capacitors may be too big.

If the power supply is AC, there's a short circuit across it through the capacitors, so if the capacitors are not small enough to filter the frequency then it won't work as you've got a short circuit = no spark. In that case you need capacitors that are small enough to filter/block the power supply frequency so it sees an open circuit, then it should produce a spark.

[edit] From the picture it looks like the power supply output is 14kHz, so you essentially need capacitors that are small enough to act as a high pass filter with a cutoff of at least 14kHz to allow the open circuit condition.

I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.

What happens if the capacitors are connected in series in test 2?

I'll give this a try.

Also, you shouldn't be doing experiments on the floor when cats are walking around

Haha I am painfully aware of that It was 4am at night and I had just finished building the circuit, so I was super eager to fire it up. I'll be more careful next time, promise


While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.

"An NST with a GFI circuit CAN NOT be used to power a tesla coil."

Could this be causing the spark not to fire?

[dR-Green]

I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.

C = 1/(2pi*F*Z)

Where
C = capacitance in Farads
F = frequency in cycles per second
And Z in Ohms is probably the impedance of the bars, but since you don't know that either I would guess that 100-500pF should do it.

The furnace transformer probably outputs 60Hz so the 2000pF easily isolates it.

While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.

That might be a secondary effect, but if there's a ground fault which would trigger it to cut out then a short circuit due to the AC passing straight through the capacitors is probably it.

[j dove]

Hairpin circuit

Hi dR-Green, thanks so much for your quick and detailed reply!



Literally only the spark gap, I disconnected the leads to the caps in this test, and the caps lead to the bars, so they are automatically also disconnected.



I attached an image of the Seletti NST I am using to this reply. It is rated at 220V AC, 50Hz, 0.42A input and 10kV, 30mA, 34kHz output. The capacitors are 40kV, 2000pF ceramic doorknob capacitors. Is there a way to calculate the required capacitor size based on power supply output frequency?

The behavior of the circuit does seem to indicate a short circuit through the capacitors, but the 10kV input in combination with these exact 40kV/2000pF caps is what Karl Palsness used in his circuit. Except he used a furnace ignition transformer instead of an NST.



I'll give this a try.



Haha I am painfully aware of that It was 4am at night and I had just finished building the circuit, so I was super eager to fire it up. I'll be more careful next time, promise


While searching for a solution, I learned that many Tesla Coil builders have problems with newer NST's (mine is brand new), because they contain a Ground Fault Interrupter (GFI) which shuts off a circuit when it senses something odd is going on.

"An NST with a GFI circuit CAN NOT be used to power a tesla coil."

Could this be causing the spark not to fire?

Hi ,

Was just reading your thread and thought that's would give my thoughts on your problem.
Though DR green has given you some very good advice here.
You can't use a newer NST as yes they do have a GFI and will not work for Tesla coil or the like. I made the same mistake as you, so is only a learning experience.
I then switched to a 10,000 volt furnace transformer and it worked quite well. I have several sets of home made high voltage capacitors that work well for it. There values are 1200 pf , 491 pf and 300 pf . All are made of aluminum and polycarbonate .

You can learn much from this design and even repeat some of Eric Dollards experiments with it. I have been able to replicate his metallic attraction with a light bulb. All different metals were used, Cu , Al, Pb, Fe, Ar, Au, Zn , all were attracted to the bulb and remained so even for a time after the power was off. Was not static electricity as I made sure to ground support system. Very strange effect, but you don't need a tesla coil to do it. Also some work with gas discharge tubes as per tesla writing that you quote in the beginning.

I would recommend to build your own capacitors as is a great learning experience and is not as easy as it sounds. I made nine of them before I got one that didn't short out, catch fire, melt or just plain fall apart. But now I know how to make very good high voltage capacitors.
Mine are several years old now are are still as good as new. And there are other experiments on can use them for in the future.

Best of luck with your experiments,


Jeff

[Kregus]

Hi Jeff and dR-Green, thank you both so much for all your valuable info! I will experiment with lower pF caps and will try to get a different type of transformer, without GFI.

I was really hoping to be able to use this off-the-shelf NST from Seletti instead of having to browse eBay for an old transformer, because this would make it a lot easier for people to replicate the device.

EDIT
I was just reading more about Capacitive Reactance, or the opposition to current flow in AC circuits, and found:

"At DC a capacitor has infinite reactance (open-circuit), at very high frequencies a capacitor has zero reactance (short-circuit)"

Since the NST I use outputs 34kHz, the short-circuit hypothesis seems extremely likely. The article also said:

"If either the Frequency or Capacitance where to be increased the overall capacitive reactance would decrease"

So this indeed means, as dR-Green mentioned, that the higher the output frequency of your power source, the smaller the Farad value of your capacitors has to be to block the AC current. However, since DC is completely blocked by capacitors, regardless of their Farad value, it might make sense to use a HV DC power source instead?

[blackchisel97]

Hi Kregus,

I have done several experiments using similar hv transformers. GFI isn't a problem but it's output frequency makes it impossible for the capacitors to charge and they will appear as a short. OBITs output 50 or 60Hz, depending on the country and capacitors can "catch up". You can overcome this problem by putting hv diode in series (between the transformer and capacitor). Diode direction doesn't matter. Also, adding small hv capacitor across the spark gap (boost cap) may improve firing. It's value can be from couple hundred pF up to 1nF. I usually use 470pF rated accordingly to the power supply being used.
OBITs are generally designed to work as igniters in oil furnaces and I would exercise caution when running it at full voltage for extended periods of time. I used OBIT in my build but added variac to regulate the output.

V

[Kregus]

Ok, so I tested the circuit with 30kV 100pF doorknob capacitors instead, but the same thing happened: the high-frequency AC current was causing a short-circuit through the capacitors, preventing the spark gap from firing.

I then built a Full Wave Bridge Rectifier to turn the HF AC into (almost) DC, using four 30kV HF diodes (see attached image).

This worked, even with the original 40kV 2000pF caps! The sound of the spark gap is now much louder, and initial testing showed nodes and anti-nodes on the copper bars.

However, when the spark gap is made too big, sparks fly off of the diodes instead of through the spark gap! This is a dangerous situation, and so to prevent my house from burning down, I think the best and unfortunately most expensive way forward is to find a 10kV 60Hz power source.

Hope this information helps someone out!

[blackchisel97]

Kregus,

You don't need a FWBR, just one HV diode to clip one half of AC. How far did you open SG electrodes?
I have 4.7nF caps in my version.

V

[Kregus]

Kregus,

You don't need a FWBR, just one HV diode to clip one half of AC. How far did you open SG electrodes?
I have 4.7nF caps in my version.

V

Thanks for the reply! Good news that my circuit can be simplified :-)

Do you mean one HV diode per wire coming from the NST (so two in total), or really just one HV diode across either of those two wires? And why is the capacitor able to charge with a HF halfwave, but not a HF fullwave AC current?

I do think a DC current might be even more convincing though to doubters, because DC should really not cross the caps.

Thanks!

[blackchisel97]

@Kregus,

Just one diode on either lead from electronic NST. Direction doesn't matter. These diodes have short bodies and may arc across at higher voltage. I used them couple times in CW Multipliers and had such problem until immersing boards in oil.
This is the one I tried in couple setups (definitely an overkill for this project) - 2CLG 30KV 2A High Voltage Diode HV Rectifier Tesla Ham | eBay

V

[Kregus]

@Kregus,

Just one diode on either lead from electronic NST. Direction doesn't matter. These diodes have short bodies and may arc across at higher voltage. I used them couple times in CW Multipliers and had such problem until immersing boards in oil.
This is the one I tried in couple setups (definitely an overkill for this project) - 2CLG 30KV 2A High Voltage Diode HV Rectifier Tesla Ham | eBay

V

Thanks for the explanation!

I ran some tests by moving a fluorescent tube along the copper bars (as described here), first with my FWBR setup, and then with your single diode solution. Here are my findings:

FWBR
The tube lit up brightly at the base of the copper bars and got less bright as I moved it up along the bars. The lamp went completely dark after I was about 60cm from the bottom (bars are 100cm long), and stayed dark until the top.

To me this indicated a single node on the bars, and I thought that by making the spark gap smaller, a higher frequency standing wave would be created, and thus more nodes would appear on the bars.

That's why I changed my 5mm spark gap down to 4mm, then 3mm, then 2mm, and finally 1mm, but nothing seemed to change. The lamp would still go dark after about 60cm and would't come back on...

Single HV diode
I then decided to perform the same experiment using the single HV diode method suggested by Blackchisel97.

Using this method, my 5mm spark gap created only one spark across the gap and then arcs at the diode, so I made the gap smaller until it kept firing properly, which was at 3mm and lower.

When I now moved the fluorescent tube along the copper bars, it remained lit equally bright from the bottom to the top. I also got lightly shocked through the glass of the bulb, so I aborted the experiment.

---

Is switching to a 60Hz power supply the only way to get this circuit to function properly?

Are my copper bars too short to have more than one node appear on them? Or is there a way to create more nodes on the bars? Because adjusting the spark gap distance did not seem to make a difference.

Any help is greatly appreciated!

[blackchisel97]

When I now moved the fluorescent tube along the copper bars, it remained lit equally bright from the bottom to the top. I also got lightly shocked through the glass of the bulb, so I aborted the experiment.

@Kregus,

Did you have both bars bridged at the top? Once it is bridged (closed circuit when SG fires) you should not feel much even, with both hands on the bars (make sure the top connection is secure if you're going to attempt such test. Otherwise, it may be a shocking experience).
This circuit is no different than HV/LF section of a resonant transformer with a single turn primary. My bars are 1.5m (5ft) tall and made sectional so I can replace straight bar with something else or make shorter. That was a few years ago when I put this together but still have it.

V

V

[mikrovolt]

It was what Tesla observed when turning off the large generators.
The spark gap functions as a trigger used long before Tesla. This article explains how
and why Tesla's spark gap included the affects attributed to abrupt discharges.
Article shows why He pushed the gap to extremes to get stinging affects and other
characteristics also showed the nature of AC high frequency at various frequencies.
This would lead experimenters to operate at different frequencies.

The question about the nature of what "disruptive discharges" do and
what affect they can produce has been a concern on the design
of Tesla circuits often fashioned to an application having dedicated components.

http://www.stealthskater.com/Documents/Tesla_10.pdf

I have attempted to draw a spark gap at the moment of spark
to show both E and H fields around the conductive streamer (spark).

spark gap.JPG

In order to explain how Tesla may have differed from Lecher and
others such as Ampere was cited to have built hairpin's in the 1840's.
There are some archives on Lecher's study that are public.
The others I could not find listed on internet search engines.

This is 3D rendering of a longitudinal wave.
longitudinal 3D.jpg

To show the Difference when compared to propagation of a Hertzian wave.
The concept leans toward the magnetic component and the E-field not shown.
The lobe forms and disconnects into the D formation.
https://www.allaboutcircuits.com/upl...ldFarField.gif
a look at Antenna propagating E and H fields.
A classic antenna film still useful for understanding concepts.
https://www.youtube.com/watch?v=eNouTh7S_ys

[mikrovolt]

Other varieties to the output by solid state power supplies having
continuous wave small gap and in the second video showing larger gap / magnetic quench.
We can see the effect of the magnet is drawing out small thin lines compared to
the large thick lines.
Third video see the bubble shape /small lines non rectified.
spark gap.JPG


https://www.youtube.com/watch?v=J-3AR6RquyM
https://www.youtube.com/watch?v=u7oziZUJy5Q
https://www.youtube.com/watch?v=OdiB4GuuSJ8

There is an opinion that a direct current impulse are quite different.
I will use this separate post to isolate this type from the other spark gaps.